Large tanks for storing chemicals, such as acids and other liquids, are used in any situation where a large quantity of such chemical is needed for a manufacturing process. For instance, large amounts of hydrofluoric acid are used in some semiconductor manufacturing processes and are required to be stored in large tanks at a manufacturing plant. Such tanks are subject to pressure from different sources. For instance, a common method of filling a tank with fluid from a source is to place the fluid under a pressure to urge or move the fluid into the tank.
Another source of pressure is due to the injection of an inert gas, such as nitrogen, above the fluid. The nitrogen is placed in the tank at a low pressure so that the fluid does not begin to vaporize. The interior of the tank is also subject to this pressure.
Furthermore, a tank maintained in the outdoors is subject to changes in pressure due to changes in ambient temperature from exposure to sunlight and wind, etc. Changes in temperature cause the pressure inside the tank to vary according to known scientific principles. If the pressure is allowed to vary past tolerable levels, the tank may be damaged or even rupture. The pressure within a sealed tank may also change depending on the volume of liquid stored in the tank. For example, if the tank is filled with too much liquid, then the pressure may increase. If liquid in the tank is withdrawn, then pressure in the tank may decrease.
One way to accommodate pressure changes in a sealed chemical storage tank is to design a stronger tank that can withstand the pressure changes. One type of high-strength tank is made of stainless steel and is lined with a refractory such as glass. Such a tank is able to withstand wide pressure variations and is usable with many chemicals. However, some chemicals, such as hydrofluoric acid, readily react with glass, and it therefore may be necessary to store such chemicals in a tank made of reinforced polyester. This type of tank will not react with hydrofluoric acid, but the tank is not as strong as a stainless steel tank.
Another way to accommodate pressure changes in chemical storage tanks is through the use of pressure relief systems. Some such systems include internal mechanisms that are prone to failure, especially when used with tanks storing corrosive liquids. Additionally, some such systems require gas to move into and out of a tank to accommodate pressure changes. However, it may be desirable to restrict the flow of gas into and out of a tank because of chemicals or contaminants that may be in the gas.
Some systems only provide vacuum relief for a tank, such as when liquid in a tank is withdrawn creating a partial vacuum. For example, SEH America, Inc. has used a system for a number of years that includes a pipe extending from the top of a chemical storage tank down to the top of a seal pot. The seal pot is a cube-shaped box made of transparent plastic that encloses a certain volume. A pipe attached to the bottom of the seal pot extends downward from the seal pot, and then turns upward to create what may be thought of as an elbow. The pipe then extends to a gas cleaning apparatus or scrubber, and then to atmosphere. A volume of liquid is placed in the elbow of the pipe so that the liquid extends slightly up into the bottom of the seal pot. When a vacuum or decreased pressure is created in the tank, the liquid in the elbow is drawn up into the seal pot. The volume of the seal pot is greater than the volume of the liquid, so the liquid does not fill the seal pot. Because the liquid does not fill the seal pot, the liquid does not enter into the pipe extending from the top of the seal pot to the tank. Instead, when the liquid is drawn into the seal pot, gas from behind the liquid is drawn through the liquid and seal pot and into the tank, thereby regulating the pressure in the tank. When the pressure is regulated, the liquid falls back into the elbow, sealing the pipe. The seal pot, elbow and volume of liquid are sized to accommodate various pressure decreases in the tank.
The system used by SEH America, Inc., however, was designed only to accommodate vacuum relief for a tank. It did not accommodate pressure increases in a tank. In that system, pressure would build in the tank until the pressure was sufficient to force the liquid up the elbow toward the gas cleaning apparatus. That pressure, however, could damage the tank.
The present invention is a system that effectively regulates both pressure increases and vacuum in a chemical storage tank. The present invention provides an effective and cost efficient way to protect such tanks, to accommodate pressure changes in a tank, and to allow for volumetric changes of gas in a tank.